This lab grows Pluto’s ices so New Horizons knows what it’s looking at

When New Horizons zips past Pluto and Charon tomorrow, scientists will interpret the worlds based on earthly ice samples grown in this laboratory.
By | Published: July 13, 2015 | Last updated on May 18, 2023
Northern Arizona University astronomer Stephen Tegler (right) and graduate student Terry Stufflebeam grow samples of Pluto’s surface in the Astrophysical Ice Laboratory, a collaboration between the school and Lowell Observatory.
Astronomy: Eric Betz

It’s tough to hear Stephen Tegler talk over the hums and hisses as a table-sized machine cranks out another slice of Pluto for testing. The Astrophysical Ice Laboratory is tucked away in a nondescript room between office spaces in the astronomy and physics department at Northern Arizona University (NAU).

And when New Horizons makes its closest approach to Pluto tomorrow morning, astronomers will analyze the data it returns thanks to the samples grown right here. Tegler and other NAU scientists work with Will Grundy, a co-investigator for the spacecraft’s infrared spectrometer and heads the surface composition team.

Pluto is an ice giant made almost entirely of frozen water. But New Horizons will be studying the thin veneer of nitrogen ice that covers Pluto’s surface, as well as the small percent of surface methane ice that mixes with the atmosphere.

In order for astronomers to understand Pluto’s surface, the lab takes gases and cools them down to some 390°  below zero at the near vacuum of space. They can also combine ices to study potential mixtures of nitrogen and methane. Once they have a sample, they examine the ices using spectroscopic instruments to understand its unique light signature.

“We’re really trying to tell how the ice changes and the effect on spectra,” Tegler says.

Those results will help the New Horizons team understand what they’re looking at on the surface of both Pluto and Charon.

“Our goal is to understand how the alien landscapes that will soon be revealed by New Horizons were created via the physical and chemical interactions of these exotic ices,” Grundy said in a recent media release.

In 1976, astronomers Dale Cruikshank, Carl Pilcher, and David Morrison found methane ice on Pluto, giving scientists their first proof that the world wasn’t rocky. And it wasn’t until 1994 though, that Hubble spotted the first-ever surface features on Pluto.

But until recent days, astronomers have relied on lab measurements to guess at what’s happening on Pluto’s surface.

“Methane should not exist on the surface of Pluto,” Tegler says. He explains that the chemical compound is easily stripped away by solar radiation, whereas nitrogen is not. And yet spectral analysis shows methane does exist on the surface.

And on Sunday night, New Horizons confirmed that Pluto has a polar cap made of methane and nitrogen. The spacecraft’s views of Charon also revealed that world has a mysterious “dark cap.”

“There must be a replenishing source [for the methane], so maybe there’s an active surface,” Tegler adds. That could take the form of cryovolcanism — or icy eruptions.

Most ice labs grow thin ice samples, which work well for compounds like methane that have strong and easily seen absorption bands. But nitrogen needs Pluto slices several centimeters thick for its much weaker bands.

“There’s only a handful of places that can grow these thick samples,” Tegler says

Based on what their instruments have found, the astronomer says it’s possible Pluto has regions with thick deposits of crystal clear ice. “I wouldn’t be surprised if the ice is very clean and very pure,” he says.

And the lab also found startling results when it looked at Pluto’s twin — or nemesis, depending on your planetary persuasion. Eris helped dethrone Pluto from the planetary family in 2006. Its size is almost identical to Pluto’s (New Horizons’ data released Monday says Pluto is just slightly larger).

By studying nitrogen and methane in the lab under both dwarf worlds’ conditions and comparing the results to telescope observations, the astronomers showed that Eris’ nitrogen was simply hiding behind that weak band. Incredibly, each dwarf planet has a surface composition of 90 percent nitrogen and 10 percent methane.

The size similarity is still a conundrum because while Pluto is slightly bigger than Eris, the smaller world is puzzlingly much more massive.

However, Eris is near aphelion — the farthest it gets from the Sun — while Pluto is close to perihelion. So Tegler wonders whether those similarities will hold as Eris makes its close approach to the Sun and Pluto retreats. Sadly, none of us will be around to find out, as Eris won’t return to perihelion for some 240 years.

Fortunately, we will get to see Pluto as New Horizons flies past tomorrow morning.

Eric Betz is an associate editor of Astronomy. He’s on Twitter: @ericbetz.